1. Field of the Invention
The present invention relates to communication terminals and methods for modulating and demodulating data for transmission/reception and, more particularly, to a communication terminal and a method for detecting a characteristic of a communication medium, changing a communication parameter for modulation/demodulation so as to cause the communication parameter to follow the state of the communication medium, and transmitting/receiving modulated/demodulated data based on the communication parameter.
2. Description of the Background Art
In general, in the case of a system where the characteristic of a communication medium is fluctuated with time (for example, a wireless communication system where multipath fading occurs or a power-line communication system influenced by group delay), the communication-medium characteristic is significantly fluctuated depending on a positional relation between a transmission terminal and a reception terminal. Therefore, in a communication system using a multi-carrier transmission scheme in which a communication parameter, such as a sub-carrier for use and a modulation scheme, is selected for communication depending on the communication-medium characteristic, the communication-medium characteristic is detected between the transmission terminal and the reception terminal for selecting a communication parameter. In this specification, a series of processes of detecting a communication-medium characteristic and selecting a communication parameter is hereinafter referred to as a communication-medium-characteristic detecting scheme. In this communication-medium-characteristic detecting scheme, the quality of a communication medium on each sub-carrier is evaluated. Therefore, the communication-medium-characteristic detecting scheme is also referred to channel estimation. Particularly, in a communication system using a multi-carrier transmission scheme, when the communication parameter is changed for each sub-carrier depending on the state of the communication medium for each sub-carrier, a deteriorated sub-carrier may be controlled so as not to be used for the purpose of preventing data from being transmitted on that deteriorated sub-carrier (for example, in Discrete Multi Tone (DMT) scheme). In such a communication scheme, a special evaluation-series packet using all sub-carrier is employed for communication-medium estimation, thereby estimating all sub-carriers and performing the communication-medium-characteristic detecting scheme (channel estimation) (for example, Japanese Patent Laid-Open No. 2002-158675).
Such a conventional method of executing the communication-medium-characteristic detecting scheme significantly depends on a characteristic unique to the communication medium (for example, Japanese Patent Laid-Open No. 2000-184061).
In a known scheme for making the method depend as less on the characteristic unique to the communication medium as possible, the communication-medium-characteristic detecting scheme is performed in predetermined cycles or upon estimation that the communication-medium characteristic is deteriorated when the number of packet retransmissions exceeds a predetermined number (for example, Japanese Patent Laid-Open Publication No. 2002-158675).
In another scheme, a transmission speed, an error rate, and a reception signal intensity are measured for determining a transmission scheme (for example, Japanese Patent Laid-Open No. 2003-209537).
FIG. 12 is a sequence diagram showing a conventional scheme for determining a timing of performing the communication-medium-characteristic detecting scheme independently of the characteristic unique to the communication medium. In FIG. 12, a transmission terminal 9000 transmits an evaluation-series packet 9001 for detecting a communication-medium characteristic to a reception terminal 9100 in order to determine a communication parameter for defining a scheme of modulating data to be transmitted to the reception terminal 9100. The reception terminal 9100 uses the evaluation-series packet 9001 from the transmission terminal 9000 to detect a communication-medium characteristic for each sub-carrier, and then transmits an evaluation-result packet 9002 containing information regarding the detected communication-medium characteristic to the transmission terminal 9000. Based on the evaluation-result packet 9002, the transmission terminal 9000 ascertains the communication quality for each sub-carrier, and determines a communication parameter for each sub-carrier. The transmission terminal 9000 then uses the determined communication parameter to modulate data, and then transmits a data sequence 9003 including a plurality of packets to the reception terminal 9100. As illustrated in FIG. 12, it is assumed herein that packet losses occur when the data sequence 9003 is transmitted. When such packet losses occur, the transmission terminal 9000 retransmits the lost packets. Unless the number of retransmissions exceeds a predetermined value, the transmission terminal 9000 transmits the data sequence without performing a communication-medium-characteristic detecting scheme until the next cycle of performing the communication-medium-characteristic detecting scheme. When the next cycle of performing the communication-medium-characteristic detecting scheme arrives, the transmission terminal 9000 transmits an evaluation-series packet 9004. Then, as described above, the transmission terminal 9000 uses an evaluation-result packet 9005 from the reception terminal 9100 to determine a communication parameter for each sub-carrier for modulation based on the communication parameter, and then transmits a data sequence 9006.
As illustrated in FIG. 12, it is assumed herein that packet losses occur when the data sequence 9006 is transmitted. When such packet losses occur, the transmission terminal 9000 retransmits lost packets. When the number of retransmissions exceeds the predetermined number, the transmission terminal 9000 determines that the state of the communication medium is deteriorated, and then transmits an evaluation-series packet 9007 for detecting the communication-medium characteristic. The reception terminal returns an evaluation-result packet 9008 to the transmission terminal 9000. Based on the evaluation-result packet 9008, the transmission terminal 9000 determines a communication parameter for modulation based on the communication parameter and transmission of the following packets.
As such, in the conventional communication systems, the communication-medium-characteristic detecting scheme is performed cyclically or when the number of retransmissions of the packets exceeds the predetermined number, thereby determining a communication parameter so as to follow the current state of the communication medium.
FIG. 13 is a graph showing a relation between a communication rate and a state of the communication medium in a conventional case where the communication-medium-characteristic detecting scheme is cyclically performed. Here, the communication rate is a communication speed calculated by the set communication parameter.
In FIG. 13, times T1011 and T1012 represent times of starting the communication-medium-characteristic detecting scheme. As illustrated in FIG. 13, in the method of cyclically performing the communication-medium-characteristic detecting scheme, the communication parameter determined at the previous time T1011 is used during a period until the next time T1012 arrives even if the state of the communication medium is fluctuated. During this period, a sub-carrier or a modulating scheme adapted to fluctuations in the state of the communication medium cannot be used. Therefore, packets transmitted from the transmission terminal cannot normally arrive at the reception terminal, thereby disadvantageously decreasing a throughput. For example, in a section P1 illustrated in FIG. 13, communication is tried at a high communication rate even through the state of the communication medium is deteriorated, thereby increasing losses of the packets to be transmitted and decreasing the throughput. Also, there is another problem in which a period from the time when the state of the communication medium is fluctuated to the time when the communication parameter is changed is long.
One way of solving these problems is to shorten the cycle of performing the communication-medium-characteristic detecting scheme so as to follow fluctuations of the state of the communication medium. In the communication-medium-characteristic detecting scheme, the reception terminal evaluates a reception CINR (Carrier to Interference and Noise power Ratio) for each sub-carrier. In order to improve the accuracy in CINR evaluation, the transmission terminal transmits an evaluation-series packet having a long data length (the number of symbols). Therefore, as the number of time of performing the communication-medium-characteristic detecting scheme is increased, the temporal proportion of the evaluation-series packet is increased in the communication system. For this reason, even if fluctuations in the state of the communication medium is followed by shortening the cycle, the throughput in the entire system is not improved.
FIG. 14 is a graph showing a relation between a communication rate and the state of the communication medium in a conventional case where the communication-medium-characteristic detecting scheme is performed when the number of packet retransmissions exceeds a predetermined value. In FIG. 14, times T1013 and T1015 represent times when the cycle of detecting the communication-medium characteristic arrives. Time T1014 represents a time when the communication-medium-characteristic detecting scheme is performed because the number of packet retransmissions exceeds a predetermined value. As illustrated in FIG. 14, in the method of performing the communication-medium-characteristic detecting scheme when the number of packet retransmissions exceeds a predetermined value, if the state of the communication medium is deteriorated, a modulation parameter is changed so as to follow the deterioration, thereby decreasing the communication rate. However, even if the state of the communication medium is changed from a deteriorated state to an improved state, communication is performed by using the communication parameter adapted to the deteriorated state of the communication medium (refer to a section P2 in FIG. 14). Therefore, this poses a problem in which, even if the state of the communication medium is improved, the throughput is not improved.
As such, conventional problems include a decrease in throughput in the entire communication system because the communication parameter cannot follow the state of the communication medium particularly when the state of the communication medium is changed from a deteriorated state to an improved state, and a long time period from the time when the state of the communication medium is fluctuated to the time when the communication parameter is changed. These problem could further pose a problem, for example, in which a user cannot continuously view video streams of satisfactory quality during a period until the communication parameter is changed to an adequate one because the state of the communication medium is deteriorated.